In light of recent earthquakes in the United States, Japan and elsewhere, considerable attention is now directed toward developing buildings that resist damage during earthquakes. Although the seismic performance of load bearing structures in buildings has improved, “non-structural” or architectural building elements have proved to be vulnerable to earthquake-induced damage. For example, curtain walls (a curtain wall is any exterior building wall comprised of any material, which carries no superimposed vertical loads and is “hung” on the building structural frame) and storefront wall systems have shown the vulnerability of architectural glass and related glazing components to damage during earthquakes. This damage includes serviceability failures (e.g., glazing gasket dislodging, sealant damage, glass edge damage and glass cracking), which require expensive building repairs and could ultimately lead to failures in the form of glass fallout, which present a life safety hazard. Earthquake-induced architectural glass glazing system failures lead to costly repairs and can impose liabilities to building designers, building contractors, building owners and insurers.
In response to concerns about nonstructural damage during earthquakes, recent model building codes, e.g., International Building Code (IBC), 2000 (ICC 2000), now require nonstructural components, such as architectural glass panels, to accommodate the maximum allowed building story drifts. According to IBC 2000, exterior nonstructural wall panels or elements that are attached to or enclose the structure shall be designed to resist the forces prescribed by an equation presented in the model building code and shall accommodate movements of the structure resulting from response to design basis ground motions. In general, seismic codes require wall systems to accommodate drift without much guidance on how to achieve “acceptable” seismic performance for various wall system types. However, as noted by Behr and Wulfert (2001), new seismic design provisions for architectural glass published in the 2000 NEHRP Provisions (National Earthquake Hazard Reduction Program 2001) are slated for adoption in the 2003 edition of the IBC. The new NEHRP seismic design provisions for architectural glass are based on a combination of design experience and laboratory test data. Moreover, these provisions now reference AAMA (American Architectural Manufacturers Association) test procedures (AAMA 2001) for determining the serviceability and glass fallout resistance of curtain wall and storefront wall system mock-ups. Although the AAMA standard test procedures do not cover wall system types other than curtain walls and storefronts, these two wall system types are prevalent in modern building practice.
Aside from those glass configurations specifically exempted from mock-up testing in the NEHRP design provisions, selection of appropriate architectural glazing configurations for seismic resistance can be a challenging and iterative process. Fortunately, a series of laboratory studies and some post-earthquake reconnaissance surveys conducted during the last twenty years have generated a significant database on the expected seismic performance of various combinations of architectural glass and wall system framing types (Memari et al. 2002a, EERI 2001, Behr 1998, Behr and Belarbi 1996, Behr et al. 1995a, Behr et al. 1995b, EERI 1995, Pantelides and Behr 1994, Lingnell 1994, Culp and Behr 1993, Wang 1992, King and Thurston 1992, Thurston 1992, Deschenes et al. 1991, Lim and King 1991, EERI 1990, Wright 1989, Evans et al. 1988, Sakamoto et al. 1984, Sakamoto 1978). Additional studies have been directed toward the development of seismic isolation methods for new wall system installations and techniques to predict and mitigate glass damage and glass fallout in existing wall systems (Memari et al. 2002b, Memari and Kremer 2001, Brueggeman et al. 2000, Memari et al. 2000, Zharghamee 1996).
Several methods are available to mitigate architectural glass damage caused by earthquakes, but there is an ongoing need to improve both the glass cracking resistance and the glass fallout resistance in earthquake prone regions and elsewhere.
One method of improving the earthquake resistance of architectural glass is to use laminated glass, which usually consists of two glass plies bonded together with a transparent polymeric interlayer such as polyvinyl butyral (PVB). Specialty laminated glass configurations are also available as glass-plastic laminates and laminates with multiple layers of glass and/or plastic, and all-plastic laminates. Laminated glass, particularly when the glass plies are made of either annealed glass or heat-strengthened glass, is highly resistant to glass fallout because any broken glass fragments remain adhered to the PVB interlayer and resist falling dangerously from the wall system glazed opening. However, individual glass plies in a laminated glass unit are still vulnerable to cracking at drift levels comparable to monolithic glass panels with square-edged corners of the same nominal thickness as the laminated glass unit. Furthermore, a cracked laminated glass unit would still need to be replaced at a significant cost. Hence, the use of laminated glass can improve resistance to glass fallout, but not the resistance to glass cracking.
Another earthquake-resistant glazing method is to apply a polymeric film such as polyethylene terephthalate (PET) over the entire glass surface and to use an appropriate anchoring technique to secure the film edges to the wall system framing. This method, like the use of laminated glass, can resist glass fallout effectively, but does not necessarily resist glass cracking. Although anchored films are used widely to retrofit in-service glass panels, application of anchored films is labor intensive, and often requires a high degree of workmanship in the film application and the film anchorage installation that is a challenge to achieve properly in the field. Unanchored films, sometimes applied as a seismic retrofit measure, are not completely effective in preventing glass fallout due to earthquake-induced building motions (Behr 1998).
For some wall system designs it is possible to use deeper glazing pockets for frame members that hold the glass, thereby providing larger glass-to-frame clearances in an attempt to avoid glass-to-frame contact during racking displacements in an earthquake. This method presumes that the glass panel will have more freedom to translate and rotate within the glazing pocket, thus avoiding early glass failure under racking conditions. This solution, however, is costly in terms of the volume of wall system materials utilized, and is not always preferred architecturally because it requires the use of wide mullions to provide the required glass-to-frame clearances needed to avoid contact. Moreover, if the glass panel is shifted too far laterally in a particular direction due to in-service conditions or faulty installation, the weather seal of the framing system can be compromised and the glass itself could be more vulnerable to cracking under subsequent wall system racking movements.
Finally, seismically isolated wall systems using unitized framing, or the recently developed “Earthquake Isolated Curtain Wall System” (EICWS) are also available. Typically, isolated wall systems are designed to accommodate in-plane racking movements, but the EICWS can accommodate movements in any direction because it permits the multidirectional sliding of the curtain wall in one story relative to adjacent stories. Although the EICWS solution is capable of providing a high level of earthquake resistance to virtually any type of architectural glass and any type of glazing system, the EICWS is designed primarily for new building construction, and, like other seismically isolated wall systems, could impose additional building design and construction costs.
Although methods such as seismically isolated wall systems, glass with anchored safety films, laminated glass, and larger glass-to-frame clearances (i.e., wide mullion designs) can be used to mitigate earthquake-induced building envelope damage, these methods have disadvantages. Specifically, due to cost and complexity, most earthquake-resistant wall systems are tailored primarily for new building construction, not building retrofits; most earthquake-resistant wall systems are significantly more expensive than conventional wall systems not designed specifically for earthquake resistance; most earthquake-resistant wall systems increase glass fallout resistance, but not all of these systems increase the glass cracking resistance; and some earthquake-resistant wall systems limit aesthetic choices in the architectural design of a building's exterior. As a result, there is an ongoing need to improve both the glass cracking resistance and the glass fallout resistance of architectural glass under earthquake loading conditions or conditions that cause such damage.